This paper presents results from an intensive long term investigation in three comparable trunk mains and downstream impact of non-invasive, in-service flow conditioning to manage discolouration risk.Findings show that flow conditioning, the careful regular increase in flows to mobilise small amounts of material from cohesive layers formed at the pipe wall, provides immediate risk mitigation and system resilience benefits. Evidence is presented showing longer term risk reduction in the trunk mains and a 25% discolouration risk reduction in the downstream networks. Whilst the flow conditioning produced an acute but short duration controlled mobilisation of material from the trunk main, longer term downstream monitoring showed reduced chronic or background material loading.It is proposed this change is due to altering the material exchange behaviour and volumes bound within cohesive layers that develop on bulk water / infrastructure interfaces. The paper provides evidence that flow conditioning is an efficient strategy to manage discolouration risk and improve consumer water quality throughout water distribution systems.

Abstract. Trunk mains are high risk critical infrastructure where poor performance can impact on large numbers of customers. Both quantity (e.g. hydraulic capacity) and quality (e.g. discolouration) of trunk main performance are affected by asset deterioration in the form of particle accumulation at the pipe wall. Trunk main cleaning techniques are therefore desirable to remove such material. However, little is quantified regarding the efficacy of different maintenance interventions or longer-term changes following such cleaning. This paper presents an assessment of quantity and quality performance of a trunk main system pre, post and for 12 months following cleaning using pigging with ice slurry. Hydraulic calibration showed a 7 times roughness height reduction after ice slurry pigging, evidencing substantially improved hydraulic capacity and reduced headloss. Turbidity response due to carefully imposed shear stress increase remained significant after the cleaning intervention, showing that relatively loose material had not been fully removed from the pipe wall. Overall the results demonstrate that cleaning by pigging with ice slurry can be beneficial for quantity performance, but care and further assessment may be necessary to realise the full quality benefits.

Simulating the long term discolouration behaviour of large diameter trunk mains can aid water utilities to understand and pro-actively manage these critical assets and mitigate a key source of customer...

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